The invention concerns an injection molding machine and an injection molding method using a physical blowing agent for the injection-molding of foamed parts.
The injection moulding methods used to manufacture moulded parts include not only compact injection moulding, but also foamed injection moulding and other special methods. Structural foam moulded parts have in contrast to compact components a sandwich structure, i.e. a more or less compact skin and a closed-cell core. They are characterized by good material properties and, in addition, are of economic interest. For example, their geometrical moment of inertia is displaced to the surface layer, thus giving them a higher specific rigidity than compact parts. Moreover, owing to their low warpage, reduced internal stresses, and few sink marks, ribbed moulded parts with sudden transitions in wall thickness can be manufactured with a high degree of dimensional stability virtually without problems. And because foaming gives rise to an internal melting pressure, dwell pressure no longer needs to be applied, with the result that large-area moulded parts can be produced with low locking pressures on smaller machines. The reduction in density leads not only to savings in raw material costs, but also to a reduced component weight. Sound and heat insulation as well as improved resistance to the manufacturing media round off the properties of foamed components.
A thermoplastic foam is generated with the aid of blowing agents which can be dosed in various ways to the polymer melt so that they mix to form a single-phase solution. Analogously to the conventional injection molding method, using standard injection molding technology, the quantity of material needed to charge the mould chamber is melted in the plasticizing cylinder. The plasticizing cylinder of a standard injection molding machine is characterized by a cylinder with nozzle and a plasticizing screw with a back flow stop. During the so called plasticizing phase, plastic granulate is processed from the feed hopper over the back flow stop to the nozzle by rotating the plasticizing screw. The plastics material is melted by heat transmitted from the walls of the plasticizing cylinder. While rotating, the plasticizing screw moves in axial direction against a defined ram pressure in the direction of the feed hopper and releases thereby volume in the screw's antechamber. After having completed the dosing phase, the material dosed in the screw's antechamber is injected in the most cases with high speed into the cavity of the tool by the axial movement of the plasticizing screw. Due to the pressure drop during the flow of the melt into the cavity, bubbles are generated by the expansion of the blowing or expanding agent. Thereby a foamed structure is developed. The foam structure is fixed by cooling or heating steps depending on the type of material used, such as elastomeric or duroplastic materials. The obtainable foam densities and the plant technology needed for the manufacture depend on the type and quantity of the blowing agent used.
There are basically two types of blowing agent, chemical and physical, whereby the difference between the two lies more in the type of dosing than in the initiation of foaming.
Chemical blowing agents are mixed in the solid state with polymer granulate and decompose under the action of heat, releasing one or more fluids in the process, in most cases nitrogen, carbon dioxide, or water. The drawbacks are the other products of decomposition that can lead to degradation of the polymer matrix, a falling off of mechanical properties, discoloration in the component, and corrosion and soiling of the mould. Furthermore, the relatively low gas output from the decomposition of chemical blowing agents achieves only limited degrees of foaming.
Fluids that are dosed directly into the polymer melt are called physical blowing agents. These can be inert gases such as nitrogen or carbon dioxide, also hydrocarbons such as pentane, as well as water. Physical blowing agents can obtain considerably higher degrees of foaming. And because there are no decomposition products, there can be no discoloration, and no detriment to mechanical properties. The drawbacks always listed in the past were the complex plant technology and the difficulty in controlling the dosed quantities owing to the unsteady state of the injection molding process.
Before a thermoplastic foam can be generated in the injection molding process, a polymer/blowing agent solution must be generated under high pressure.
Here, the expanding fluid is brought into contact with the low-viscosity polymer. Depending on the process conditions, diffusion processes then take place, leading to the absorption of the blowing agent in the melt. After enough time has passed, a single-phase polymer/blowing agent solution has formed.
The methods used today differ greatly in the way they bring the expanding fluid into contact with the melt.
One way of achieving a highly uniform solution of blowing agent in the polymer is to charge the material with expanding fluid beforehand. Here, a high-pressure gasification system is used to charge the polymer with carbon dioxide before the polymer is processed. The plastic granulate is pressurized to a predefined value with CO2 in an autoclave at room temperature, whereby the polymer absorbs gas owing to the difference in concentration and pressure. The concentration of gas in the polymer is a function of the gasification time, amongst other factors. Once the saturation concentration has been reached, the pressure is reduced to the ambient value, and the gas-charged polymer is dosed to the injection molding machine via the feed hopper. The material is then melted and homogenized in the plasticizing cylinder, whereby the rise in pressure causes the dissolved gas to lie along the cylinder. On leaving the nozzle, the polymer foams as a result of the rapid drop in pressure.
The drawbacks with this method of precharging in autoclaves lie in the batch charging of the granulate (making it inappropriate for industrial applications) and the lack of flexibility with respect to time (the blowing agent diffuses continuously out of the polymer). So this method does not find practical application in industry.
Another method makes it possible to dose the blowing agent directly in the screw's antechamber. Underlying this method is a special gas injection nozzle flanged between the cylinder and the mould injection nozzle (DE 198 53 021 A1). The heart of this gas injection nozzle is formed by an annulus of porous or gas permeable sintered metal through which the melt passes during the injection process. A torpedo centered in the melt channel divides the melt upstream of the annulus for the best fluid engineering properties and recombines the two flows without dead corners once they have passed the annulus. If necessary, static mixer and shear elements can be installed for the homogeneous distribution of the polymer/blowing agent solution. The gas is fed by a gas dosing station that can regulate the mass flow to vary the proportion of blowing agent in the melt, thus obtaining various degrees of foaming.
Another technology is based on the injection of a physical blowing agent into the plasticizing cylinder of an injection molding machine (EP 0 952 908 A2). Here, the blowing agent is injected through several axially arranged radial apertures in the plasticizing cylinder's melt chamber. Upstream of each of these apertures is a controllable valve that can open and close the connection to the blowing agent supply. A cascade controller then correlates the controlled valve states with the position of the screw during the dosing process, i.e. the valves are opened and closed in succession. The purpose is to obtain the most uniform injection of the blowing agent as possible into the melt. Long mixing zones then homogenize the blowing agent/polymer solution, which in the ideal case is ready for the mould injection process as a single-phase substance.
Another process for the foaming of plastic parts implies the injection of a blowing agent in the area of an extruder (EP 1 072 375 A2). The blowing agent is added to the molten polymer through a porous region in the screw of the extruder. This process is used predominantly for the continuous production of plastic profiles by an extrusion process. To allow for injection molding type applications, such an extruder has to be combined with a plunger injection unit. The blowing agent is added in the extruder. In order to adapt this solution to a batch process, requires increased machinery and plant equipment alongside with the need for a special machine adapted specifically to the requirements.
The drawbacks of this method with the gas injection nozzle lie in the homogenization of the melt. During the mould injection process, there is, of course, only this injection time available to charge the melt with the blowing agent. And because foamed injection molding applications generally require a high mould injection speed in order to generate a high nucleation density, these times may not be long enough—despite the short diffusion paths in the annulus—to obtain an adequate and homogeneous sorption. On the other hand, feeding the blowing agent during the dosing phase is also problematic because the volume of the melt in the annulus is given by the geometry. So if the volume of the injection molded part is larger than the volume of the annulus, then this difference in volume is not charged with the blowing agent, and the molded part's foam structure will become inhomogeneous. If, on the other hand, the volume is smaller, then a part of the mass will be charged during two successive dosing phases. This is not associated with any problems as long as a single dosing cycle is adequate to enrich the melt with the blowing agent in the saturation state. In addition, static mixing elements involve the risk of damage to the polymer matrix caused by friction heat when high shearing forces act on thermally sensitive materials. Owing to the high pressure during the mould injection process a dosing station is needed to compress the expanding fluid to the appropriate high value.
The main drawback of this variant method injecting gas into the plasticizing cylinder is that a special injection molding machine becomes necessary. The cascade control system requires a correlation interface with the machine controller, as is not the case with customary machines. Because the melt is enriched with the blowing agent during the dosing phase, in the course of which the screw completes its axial travel back, several gas injection ports with the correspondingly complex valve technology are needed to obtain an approximately uniform dosing of the blowing agent in the melt. This complex plant technology does not only require high investment costs; it is a well-known fact that the more complex the applied technology becomes, the greater its susceptibility to malfunctions in the production run, and the higher the servicing costs. Guaranteeing that the blowing agent can disperse adequately in the melt after the point injection requires unusual mixer zones, which in turn require a special screw with long mixing elements and, as a consequence of the longer screw lengths, a special drive unit as well. Also needed is a dosing unit to inject the expanding fluid.